With the increasing movement of solid state electronics from discrete component usage to integrated circuitry, such as ASICs (Application Specific Integrated Circuits), advantages have been gained in miniaturization, but certain limitations have been encountered as well. In prior art, amplification systems were built with discrete components such as capacitors and resistors being used in conjunction with high gain integrated circuit amplifiers to produce various amplifier configurations such as transconductance amplifiers. The intrinsic properties of the external components as a function of the operating voltage being used was seldom a problem, if the components were not used outside of their design voltage limitations.
The same is not necessarily the case for fully integrated circuit amplifiers where devices such as capacitors are integrated onto the same substrate as the amplifier. While the various methods of integrating capacitive devices is well known to those skilled in the art, it is also known that the parasitic capacitance of these devices to the common substrate varies with their dc-biasing. In a typical high-capacitance implementation, the capacitor is constructed by placing a polysilicon layer over an n-well region, whereby the polysilicon layer forms the top plate of the desired capacitor and the n-well forms the bottom plate of the desired capacitor. The n-well, however resides atop of the common p-type substrate that is shared with the rest of the integrated amplifier circuit. As a result,
a parasitic capacitance from the n-well back plate to (grounded) substrate exists similar to that of a reverse biased pn-junction. This results in a parasitic capacitance to ground that varies inversely with the dc voltage present on the back plate of the capacitor. Where such a capacitor is used as a high pass filter, for instance, its impedance would change as a consequence of changes in the biasing voltage on the back-plate of the capacitor, thereby changing the frequency response characteristics of the amplifier. For stability in capacitance it is desirable to stabilize the bias voltage on the device. Where there is sufficient power availability to support the losses of a voltage regulator, the capacitance could be stabilized by operating the amplifier from a regulated supply voltage. Unfortunately, for battery operated devices, only a small amount of power may be available, either because of battery size limitations, or other power usage requirements, and a voltage regulator may prove impractical by reducing battery operating time, or increasing the current loading. While switching regulators may be more efficient than other regulator types, they have a tendency of introducing electrical noise and are considered impractical for small integrated low-noise amplifiers. Accordingly it does not seem to be practical to control the bias voltage on the back-plate of the capacitor and another solution is required.